Acoustic resonators can be used to implement signal processing functions in various electronic applications. For example, some cellular phones and other communication devices use acoustic resonators to implement frequency filters for transmitted and/or received signals. Several different types of acoustic resonators can be used according to different applications, with examples including surface acoustic wave resonators (SAW) and bulk acoustic wave (BAW) resonators, such as thin film bulk acoustic resonators (FBARs), coupled resonator filters (CRFs), stacked bulk acoustic resonators (SBARs), double bulk acoustic resonators (DBARs), and solidly mounted resonators (SMRs). An FBAR, for example, includes a piezoelectric layer between a bottom (first) electrode and a top (second) electrode over a cavity. BAW resonators may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs operating at frequencies close to their fundamental resonance frequencies may be used as a key component of radio frequency (RF) filters and duplexers in mobile devices.
An acoustic resonator typically comprises a layer of piezoelectric material sandwiched between two plate electrodes in a structure referred to as an acoustic stack. Where an input electrical signal is applied between the electrodes, reciprocal or inverse piezoelectric effect causes the acoustic stack to mechanically expand or contract (oscillate) depending on the polarization of the piezoelectric material. As the input electrical signal varies over time, expansion and contraction of the acoustic stack produces acoustic waves that propagate through the acoustic resonator in various directions and are converted into an output electrical signal by the piezoelectric effect. Some of the acoustic waves achieve resonance across the acoustic stack, with the resonance frequency of the piezoelectric layer being determined by factors such as the materials, dimensions, and operating conditions of the acoustic stack. These and other mechanical characteristics of the acoustic resonator determine its frequency response.
As indicated above, acoustic resonators (and other microelectronic devices including piezoelectric components) are typically applied to a substrate or wafer, and may ultimately be incorporate into a packaged unit. The substrate in particular may be subjected to external or internal forces that cause flexing or bending of the substrate and/or other portions of the package unit. For example, the different materials used to form the substrate and the electrical contacts or other components may have different temperature expansion and contraction characteristics, resulting in different rates of expansion and contraction in response to temperature changes, resulting in forces that may bend the substrate, resulting in various mechanical stresses on the acoustic resonators (or other microelectronic devices including piezoelectric components). The mechanical stresses, in turn, may cause undesirable operational changes (such as changes to resonance frequency) and/or physical changes (such as cracking or weakening of piezoelectric material). Accordingly, there is a need for providing stress relief for microelectronic devices subject to force induced stresses.